1. Field of the Invention
The present invention relates to an image pickup apparatus for picking up an image of an object.
2. Related Background Art
Conventionally, a fingerprint recognition apparatus is available that is used for fingerprint verification when a finger is positioned so as to directly contact a two-dimensional semiconductor sensor (an electrostatic type or an optical reading type). This fingerprint recognition apparatus has recently become very important in the company and individual security fields.
If the size, weight and price of the fingerprint recognition apparatus can be reduced, dramatic growth is expected in the market for business transactions for which portable telephones or other portable devices are employed.
The recognition accuracy of the electrostatic type sensor is low, however, when a finger is too dry, and attention has been drawn to the optical type because it provides excellent recognition accuracy for various finger types. An optical type fingerprint recognition apparatus is described in detail in Japanese Patent No. 310126.
Sensors used for the optical type apparatus are an area sensor shown in FIG. 1 and a line sensor shown in FIG. 2. In the above example, the optical sensor emits an LED light to irradiate a finger, and employs scattered LED light, which passes through the finger, to read as an image the surface roughness of the finger.
According to the fingerprint recognition apparatus that uses the area sensor, when a finger 1 is pressed against an area sensor 2, in the same manner as a seal, an LED is turned on and reads a fingerprint image.
According to the fingerprint recognition apparatus that uses the line sensor, a finger 1 is scanned by a line sensor 4 which includes several tens of pixel arrays, and an image of a fingerprint is read as a sensor signal.
For either type, the sensor output signal is processed by an image processing unit 5 to generate a fingerprint pattern that is compared by a pattern recognition unit 6 with a reference fingerprint pattern held in temporary storage. In this manner, fingerprint recognition can be performed.
In the image reading examples shown in FIGS. 1 and 2, the LED light is emitted to radiate the finger. Generally, equipment in which the fingerprint recognition apparatus is mounted is used inside under fluorescent light that commonly has a brightness of several hundred lx or so, or is used outside under the sun which has a brightness of at least several tens of thousands of lx. In order for an image of a fingerprint to be accurately read under these conditions, appropriate optical and sensitivity designs are required for a sensor.
Further, since for a sensor that is to be mounted in a portable apparatus a low price and low power consumption are demanded, a reduction in the size of the sensor and an increase in its sensitivity are required. However, to accurately read the image of a fingerprint, an LED whose preferable emission wavelength is 800 to 950 nm is required, and such an LED is very expensive and consumes a large amount of power.
Thus, a highly sensitive sensor is needed to reduce the required number of expensive LEDs and the LED driving current. However, when the sensitivity of a sensor is high, the sensor tends to become saturated. And since a reduced exposure time is needed to prevent the sensor from becoming saturated, an electronic shutter is required to attain a reduced exposure time. In this case, the following new problem is encountered.
A flicker is generated when an electronic shutter is driven for a short period of time under fluorescent light. An explanation for the flicker will now be given while referring to FIGS. 3 and 4.
In FIG. 3, as is well known, flicker of a period of 10 ms is generated by fluorescent light. Thus, when the exposure period of a sensor is several tens of ms or longer, variances in the integral values light amount are reduced. For example, when the length of an exposure period is 100 ms, as shown in FIG. 3, exposure variance is not a problem for a fingerprint sensor. When, however, as is shown in FIG. 4, rolling shutter driving, at a flicker period of 10 ms, is performed to expose sequentially an individual pixel row, a large flicker is generated in the output signal of the sensor.
Further, even when whole pixels are simultaneous exposed, if an image is read while a flicker is generated for a small amount of light, the SN ratio of the fingerprint image is greatly degraded and fingerprint recognition is impossible.
An explanation will now be given for outside use. Since an area sensor at least can obtain a plurality of images when a finger is pressed against it, feedback of the exposure condition can be performed and the sensor can be prevented from being saturated. However, for a scan type sensor, while multiple scanning determines the exposure condition, it is difficult to determine the exposure condition when only one scanning is preformed. This, therefore, is a serious problem, since the sensor becomes saturated and recognition of the fingerprint is not possible.